WO 90/01521 in the name of Battelle Memorial Institute published Feb. 22, 1990 discusses the polymerization of polylactic acid while maintaining residual monomer in the polymer to act as a plasticizer. The disclosure is of interest in that there is a fairly lengthy discussion of the prior art processes used to prepare polylactic acid. Most of the processes described as prior art disclose polymerizing lactic acid or lactide. There is no strong discussion regarding the preparation of lactide.
European Patent Application 0 261 572 in the name of Boehringer Ingelheim KG published Sep. 16, 1987 discloses a process in which polylactic acid is heated in the presence of a catalyst, and under vacuum, at temperatures from 130.degree. to 230.degree. C. and lactide is distilled off. The polylactic acid should have a molecular weight from 400 to 2,000, preferably from 500-800. In the polymerization of lactic acid, lactic acid is first heated. Initially low molecular weight polylactic acid forms. However, as the reaction proceeds the low molecular weight polylactic acid begins to depolymerize and form lactide. Then an equilibrium is established between lactide, lactic acid, water and low molecular weight polylactic acid. The result is that it is very difficult to directly polymerize high molecular weight polylactic acid from lactic acid. Rather, one must first prepare lactide and then subject it to a ring opening polymerization. However, the above noted equilibrium makes it difficult to produce a relatively pure stream of lactide from an equilibrium mixture.
Lactide may be in the form of several different isomers. It may be an l-, or m-isomer. In preparing polylactic acid it is generally desirable to have a polymer containing a high amount of l-isomer. The physical properties of the l-isomer and particularly the heat distortion temperature under load are better for this type of polymer.
U.S. patent application Ser. No. 659,567 filed Feb. 22, 1991, (corresponding to Canadian Patent Application No. 2,056,549 filed Nov. 28, 1991), now U.S. Pat. No. 5,136,017, discloses a continuous process for the polymerization of lactide to polylactic acid. The patent does not disclose any process for the purification of lactide to the degree necessary to polymerize lactide to produce high molecular weight polylactic acid.
Generally any prior art the applicants have been able to locate on this subject matter suggest purification of lactide by recrystallization or precipitation from a suitable solvent such as methanol. The difficulty with such a procedure is that there tends to be a commercially unacceptable loss of starting material and additionally it requires the handling and recovery of a large quantity of volatile solvents.
Accordingly there is a need for a better process for the purification of crude lactide.
The present invention seeks to overcome this need by providing a process for the melt recrystallization of lactide.